Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the chicken breasts and fry over a high heat until browned on both sides. Remove from the pan and transfer to a plate. Add the onion to the pan and fry, stirring, for 5 minutes or until lightly browned.
Stir in the garlic and flour, then mix in the stock and lemon wedges. Season with salt and pepper and bring to the boil.
Put the chicken breasts in the slow cooker pot, pour the hot stock mixture over them and press the chicken below the surface of the liquid. Cover with the lid and cook on high for 3–4 hours.
Add the pak choi and sugar snap peas and cook, still on high, for 15 minutes or until just tender. Lift out the chicken, slice the pieces and arrange them on plates. Stir the crème fraîche and herbs into the sauce, then spoon it and the vegetables over the chicken. Serve with couscous mixed with finely chopped tomato, red onion and red pepper.